In recent years, in order to smoothly use satellite broadcasting, CATV broadcasting, and VOD broadcasting, the spread of a passive optical network based on a radio frequency (RF) (radio frequency over glass: RFoG) and a development of a fiber-to-the-home (FTTH) system based on the RF are actively made. The RFoG replaces a coaxial portion in a hybrid fiber coax (HFC) network with a single-fiber passive optical network (PON), and the FTTH system used in the passive optical network is advantageous in that it allows subscribers to use their own cable modems, set-top boxes, and the like without change.
In general, the passive optical network has a structure in which a plurality of house-optical network units (hereinafter, referred to as ‘R-ONU’) disposed at a plurality of subscribers' side are coupled to one optical core, and a laser diode included in each R-ONU uses a burst mode for uplink optical signal transmission. That is, only in a case in which the RF signal is input to the R-ONU from the cable modem or the set-top box of the subscriber, the R-ONU is operated to transmit the optical signal to a central station side from the subscriber side, and in the opposite case, since the R-ONU is not operated, the uplink optical signal is not transmitted.
In a case in which the RF signal is simultaneously input to the plurality of R-ONUs, however, the RF signal activates the laser diode to generate an optical signal having a specific wavelength from the activated laser diode, and the generated optical signal is simultaneously transmitted to the central station side from the plurality of R-ONUs. When the uplink optical signal having the same wavelength is simultaneously transmitted from the plurality of R-ONUs as described above, a wavelength overlap phenomenon called optical beat interference (OBI) may occur, which results in a quality degradation phenomenon of the uplink signal. That is, in a case in which a plurality of optical signals having wavelengths which are very close to each other at the same time overlap within a single optical fiber, a noise level of an uplink receiving terminal may be increased and a network service itself may become impossible.